1. Field of the Invention
This invention relates to the fabrication of shaped laminated safety glass windshields, particularly of the type having one or more holes adjacent to but completely spaced from the marginal edge thereof. Each hole is needed to insert a shaft for a windshield wiper so that the wiper blade can operate against an outer surface of a windshield while the windshield wiper actuating device is supported within the interior of the vehicle.
In the past, the shafts for windshield wiper blades extended entirely outside the windshield. Recent efforts to improve operating efficiency of automobiles has resulted in reducing the size of automobiles. The proportional amount of glass in an automobile has increased and the space available for various automobile accessories has been compressed. Therefore, consideration has been given to extending the shafts for actuating operation of windshield wipers through apertures in the lower portion of the windshield. This raises the problem of developing a technique for making bent laminated windshields having a pair of glass sheets with aligned holes or apertures to receive the shafts therethrough.
It is well known that glass sheets that are apertured are weaker than glass sheets that are of non-perforated construction. Drilling a hole in glass weakens the glass structurally and in the past, it has been considered necessary to strengthen apertured glass sheets by thermal tempering in order to restore some of the strength lost by drilling holes through the glass. While thermal tempering improves the overall strength of glass sheets that successfully sustain the tempering treatment, drilling holes in glass causes fissures which may extend into vents, which are origins of glass breakage when the glass is chilled rapidly after being heated to its tempering temperature. Glass is notoriously weak in tension and, temporary tension stresses are established during the rapid cooling step that causes a high rate of loss during fabrication. Apertured glass sheets that survive the temporary tensile stresses during tempering develop permanent stress patterns that incorporate a compressive stress in the skin of the glass that protects an interior stressed in tension. While monolithic glass sheets that are drilled and tempered have found use in tempered automobile sidelights for automobiles and as a component laminated to undrilled glass or plastic sheets in laminated transparencies for aircraft, glass sheets drilled with apertures have never been laminated to one another to provide a bent laminated windshield with the apertures aligned through the thickness of the bent laminate after bending, to the best of our knowledge.
It is believed that the reason that such laminates have not been known prior to the present invention is because laminated glass windshields for automobiles (particularly those composed of glass sheets having a thickness per sheet not exceeding about 110 mils (2.8 millimeters), which are usually found in present day windshields, must be heated in the vicinity of the apertures to a maximum temperature not exceeding 1150.degree. F. (621.degree. C.) and usually is heated to a maximum temperature within the range of 1120.degree. F. (604.degree. C.) to 1150.degree. F. (621.degree. C.) in a sag bending operation at which a pair of glass sheets are aligned and bent by gravity sagging to conform to a mold. Such temperatures are insufficient to heal most fissures that form during drilling. In contrast, glass sheets to be tempered are heated to the range of 1200.degree. F. (649.degree. C.) to 1250.degree. F. (677.degree. C.), at which temperature sufficient healing of fissures takes place to reduce the number of vents produced from the fissures during the cooling step of tempering. Consequently, it was not feasible to both control the shape of drilled glass sheets and also avoid breakage during bending.
2. Description of the Prior Art
U.S. Pat. No. 3,150,950 to English indicates a method of drilling holes through tempered glass sheets by inducing temporary stress that temporarily counteracts the major portion of a permanent stress which makes drilling and edging of tempered glass very dangerous. The difficulty of establishing a temporary stress pattern has prevented this invention from being used outside the laboratory.
U.S. Pat. No. 3,231,352 to Carson and Leflet applies a stress pattern along the edge of a pair of glass sheet blanks during its bending and subsequently cuts the stressed and shaped glass along lines that have been stressed in compression so as to minimize the danger of damage to the glass edge during cutting and to obtain an edge stressed in compression after cutting.
U.S. Pat. No. 4,023,945 to Boaz discloses a technique of cutting an opening through the thickness of an apertured glass sheet and extending a relief opening from the aperture in the glass to the edge of the glass in order to relieve stresses associated with the glass during its tempering. The stresses developed during tempering of apertured glass sheets have caused considerable breakage without the stress-relief opening. The stress-relief opening is alleged to reduce the frequency of breakage during fabrication by thermal tempering, but causes a discontinuation in the marginal edge of the glass sheet, which weakens the glass edge and makes it more difficult to handle the glass sheet without causing danger to those handling the glass edges.
When glass sheets are bent for subsequent lamination, they are usually bent in pairs such as disclosed in the aforementioned U.S. Pat. No. 3,231,352 to Carson and Leflet, and then the bent sheets are assembled on opposite sides of a sheet of interlayer material and the assembly laminated.
It is well known to laminate glass sheets that have not been provided with apertures. In one method, disclosed in U.S. Pat. No. 2,948,645 to Keim, an assembly of two glass sheets and an intermediate layer of plastic is enclosed within a marginal evacuation chamber composed of a flexible channel-shaped member made of a fluid impervious material. The interfacial surfaces between the assembled two glass sheets and plastic interlayer are evacuated through the marginal evacuation chamber while heat is applied until the glass sheet is preliminarily bonded to the marginal portion of the interlayer at least. The preliminary pressing performed by the peripheral evacuation technique is accomplished while the assembly is exposed to atmospheric pressure. The peripheral evacuation chamber is removed and the glass/plastic assembly is finally laminated under elevated temperature and pressure conditions in an autoclave.
Another well known technique for laminating glass and plastic sheets involves the insertion of an assembly comprising a pair of bent sheets of matching outline and contour assembled on opposite sides of a sheet of interlayer material including one or more glass sheets, one of which glass sheets may be apertured, within a laminating bag or envelope, which is evacuated and sealed. A typical laminating bag suitable for inserting such assemblies for lamination contains an outer layer of a composition from the class consisting of condensation products of ethylene glycol and terephthalic acid and oil impervious nylon films and an inner-layer of polyethylene, which is preferably provided with an embossed inner surface. A method of using such a laminating bag to laminate glass/plastic assemblies is disclosed in U.S. Pat. No. 3,311,517 to Keslar and Rankin. The evacuated and sealed envelope is inserted within an autoclave for final lamination under elevated temperature and pressure conditions. After sufficient time of exposure to elevated temperature and pressure conditions, the bag or envelope is opened and the laminated assembly is removed.
Still another commercial method for laminating glass/plastic assemblies comprises a roller prepressing step followed by final autoclave lamination. The roller prepressing step may be performed using the roll prepressing apparatus disclosed in U.S. Pat. No. 3,351,001 to Aachio.